He fell from the first floor to the ground outside? I would watch him very closely, and if he isn’t getting better, then he needs to see a vet. Even if he didn’t break anything, he could have swelling and inflammation that medication could help while he heals. Especially if he is mouth-breathing because no air is coming through his nose.

Just lots of reassurance, patience, and cuddles until he forgets what happened to him. I would close windows, have screens checked, or whatever it is you need to do to make sure this kitten cannot fall from a long distance. A kitten, just like a human toddler, cannot know that walking off the edge of something is going to result in an injury or death–that’s OUR job to protect them from themselves. In time, he should calm down and once he feels better, he will be his playful self again.